This invention relates to drive systems and more particularly serpentine accessory drive systems for automotive vehicles.
These systems are in common use to transfer power from an internal combustion engine crankshaft to accessory components typically including an alternator (generator), water pump, oil pump (power steering), air conditioning compressor (via electromechanical clutch). These components are usually fixed position mounted and utilize an automatic belt tensioner to provide constant belt tension and take-up of belt slack.
Internal combustion engines generate rotating power at the crankshaft only when a combustion event occurs. This, in effect, is a pulsed system whereby the closer the spacing of the combustion events, the smoother the rotational consistency of the engine. For each combustion stroke, the crankshaft will exhibit acceleration, then deceleration, until the next combustion stroke. In general, the slower the rotation of the engine, and the lesser number of cylinders (combustion events per revolution of crankshaft) tends to increase the magnitude of the pulsing effect. Fuel burn characteristics also have a substantial influence, for example, the instantaneous acceleration of the crankshaft on a diesel engine is much greater than a similar gasoline fueled engine due to the combustion process itself.
In terms of the serpentine belt system, the crankshaft pulses are transferred to the belt as fluctuations in velocity. The velocity fluctuations of the engine are thus transferred to all the driven components in the system. Dynamic belt tension fluctuation is generated by the velocity fluctuation. Without considering the dynamic loading of the accessory components and consequent tension effects, it is obvious that the driven inertias will generate dynamic tensions as the belt continuously attempts to accelerate and decelerate such components. The magnitude of force required is proportional to the inertia and to the driven ratio. The function is squared.
Where the engine is smaller, four or five cylinders, and in the lowest speed ranges (idling area) the dynamic tension fluctuation is at the highest magnitude. The magnitude can be further increased by technological differences that serve to lower the engine rotating inertia (dual mass fly wheel) or increase the instantaneous acceleration (diesel, higher compression, etc.). Operating conditions can also have a significant effect, for instance, "lugging" where the engine is running below its ideal minimum speed (idle) at high power levels that attempt to increase the speed back to idle.
Under these circumstances, the dynamic belt loading can be so great that the belt tensioner cannot accommodate all the dynamic fluctuations. The results can include belt noise, belt slippage and forced vibration of the belt, tensioner and accessory components. Ultimately, durability is compromised.
It is possible to resolve this problem using a torsional isolator at the crankshaft, provided it has low stiffness. Such conventional torsional isolators have been employed for many years, but are bulky, costly, heavy, and display limited effectiveness. This limited effectiveness is generally the result of the drive having to be designed to carry the full power capability of the system, while rarely ever requiring the same. Thus, torsional isolators are typically too stiff.
Commonly assigned U.S. Pat. No. 5,156,573 ("the '573 patent"), hereby incorporated by reference, discloses a serpentine drive system for an automotive vehicle which provides a coil spring and one-way clutch mechanism between the alternator pulley and mounting hub structure. The disclosed preferred embodiment of the mechanism takes the form of a generally helical coil of spring steel, which performs the dual function of 1) resiliently transmitting driven rotational movements of the alternator pulley to the hub such that the alternator shaft is rotated in the same direction as the pulley while being capable of instantaneous relative resilient rotational movements in opposite directions with respect to the pulley during driven rotational movement of the pulley, and (2) decoupling the alternator pulley from the hub so that the hub structure and hence the alternator shaft can rotate at a speed in excess of the rotational speed of the alternator pulley when the speed of the engine output shaft is decelerated to an extent sufficient to establish the torque between the alternator pulley and the hub structure at a predetermined negative level.
Each of the two functions noted above has different engineering requirements for optimizing the system. For example, the resilient coupling function would optimally have a greater spring rate (a stiffer spring) than the spring rate utilized for performing the coupling/decoupling function. Optimally, a higher spring rate is desirable for transmitting driven rotational movement of the alternator pulley to the hub structure in order to accommodate the relatively high torsional forces, while a lower spring rate is desirable for the de-coupling function so that less force is exerted and thus less frictional wear and heat is generated by the mechanism during the de-coupling or overrun condition. Increasing the spring rate of the mechanism to accommodate the torsion transmitting function would be to the detriment of the coupling/de-coupling function, while decreasing the spring rate to accommodate the coupling/de-coupling function would be to the detriment of the torsion transmitting function. As another example, the coupling/de-coupling function ideally requires a material having a higher coefficient of friction than than required for the torsion transmitting function.
It is an object of the present invention to provide an improved serpentine belt drive system which individually optimizes the two functions noted above. In accordance with this object, the present invention provides a serpentine belt drive system for an automotive vehicle comprising a drive assembly including an internal combustion engine having an output shaft with a driving pulley thereon rotatable about a driving pulley axis. A sequence of driven assemblies each has a driven pulley rotatable about an axis parallel with the driving pulley axis and a serpentine belt mounted in cooperating relation with the driving pulley and with the driven pulleys in a sequence which corresponds with the sequence of the driven assemblies when related to the direction of movement of the belt to cause said driven pulleys to rotate in response to the rotation of the driving pulley. The sequence of driven assemblies includes an alternator assembly including an alternator shaft mounted for rotation about a shaft axis. A hub structure is fixedly carried by the alternator shaft for rotation therewith about the shaft axis. A spring and one-way clutch mechanism couples the alternator pulley with the hub structure. The spring and one-way clutch mechanism comprises a resilient spring member separately formed from and connected in series with a one-way clutch member. The resilient spring member is constructed and arranged to transmit the driven rotational movements of the alternator pulley by the serpentine belt to the hub structure such that the alternator shaft is rotated in the same direction as the alternator pulley while being capable of instantaneous relative resilient movements in opposite directions with respect to the alternator pulley during the driven rotational movement thereof. The one-way clutch member is constructed and arranged to allow the hub structure and hence the alternator shaft to rotate at a speed in excess of the rotational speed of the alternator pulley when the speed of the engine output shaft is decelerated to an extent sufficient to establish the torque between the alternator pulley and the hub structure at a predetermined negative level.
It is a further object of the invention to provide a device which addresses the issues noted above and can be used to transmit movement from a belt driven by an output shaft of an engine to a shaft of an auxiliary component to be driven. The device comprises a hub structure, a pulley member, and a spring and one-way clutch mechanism. The hub structure is constructed and arranged to be fixedly carried by the shaft for rotation therewith about a shaft axis. The pulley member is mounted on the hub structure and constructed and arranged to engage the belt and be rotatably driven thereby. The spring and one-way clutch mechanism couples the pulley member with the hub structure. The spring and one-way clutch mechanism comprises a resilient spring member separately formed from and connected in series with a one-way clutch member. The resilient spring member is constructed and arranged to transmit the driven rotational movements of the pulley member to the hub structure such that the shaft is rotated in the same direction as the pulley while being capable of instantaneous relative resilient movements in opposite directions with respect to the pulley during the driven rotational movement thereof. The one-way clutch member is constructed and arranged to allow the hub structure and hence the shaft to rotate at a speed in excess of the rotational speed of the pulley when the speed of the driven pulley is decelerated to a predetermined extent.
It is a further object of the present invention to provide a serpentine belt drive system in which optimizes the spring rates discussed above. In accordance with this object, the present invention provides a serpentine belt drive system for an automotive vehicle comprising a drive assembly including an internal combustion engine having an output shaft with a driving pulley thereon rotatable about a driving pulley axis, a sequence of driven assemblies each having a driven pulley rotatable about an axis parallel with the driving pulley axis, and a serpentine belt mounted in cooperating relation with the driving pulley and with the driven pulleys in a sequence which corresponds with the sequence of the driven assemblies when related to the direction of movement of the belt to cause the driven pulleys to rotate in response to the rotation of the driving pulley. The sequence of driven assemblies includes an alternator assembly including an alternator shaft mounted for rotation about a shaft axis. A hub structure is fixedly carried by the alternator shaft for rotation therewith about the shaft axis. A spring and one-way clutch mechanism couples the alternator pulley with the hub structure. The spring and one-way clutch mechanism comprises a resilient spring portion disposed in series with a one-way clutch portion, the resilient spring portion having a torsional spring rate at least ten times greater than a torsional spring rate of the one-way clutch portion. The resilient spring portion is constructed and arranged to transmit the driven rotational movements of the alternator pulley by the serpentine belt to the hub structure such that the alternator shaft is rotated in the same direction as the alternator pulley while being capable of instantaneous relative resilient movements in opposite directions with respect to the alternator pulley during the driven rotational movement thereof. The one-way clutch portion is constructed and arranged to allow the hub structure and hence the alternator shaft to rotate at a speed in excess of the rotational speed of the alternator pulley when the speed of the engine output shaft is decelerated to an extent sufficient to establish the torque between the alternator pulley and the hub structure at a predetermined negative level.
Another object of the invention is to provide a spring and one-way clutch mechanism having a clutch portion with a greater coefficient of friction than the spring portion.
Another object of the invention is to provide a spring and one-way clutch mechanism in which the clutch portion expands radially outwardly and is thus assisted by centrifugal force when engaging the alternator pulley to be coupled.
This invention is based on the fact that the effective inertia of the alternator is by far the largest in a typical accessory drive system, but uses only a portion of the power requirement for the system. If the apparent inertia can be reduced, the dynamic tension fluctuation can also be greatly reduced. By providing an effective decoupling function between the alternator pulse and the alternator rotor (armature), the apparent inertia can be significantly reduced.
It is important to note that the decoupler resilience or elasticity must be sufficiently soft so that amplification of velocity fluctuation at the pulley cannot be transmitted to the rotor in the normal operating speed range of engine where maximum dynamic tension control is desired.
The present invention provides a torque sensitive one-way clutch connected in series with a separate decoupling resilient or elastic element. It will be shown that the one-way clutch provides additional value in solving other problems, while performing its major function of maximizing the durability of the resilient or elastic decoupler.
At higher than idle operating speeds, a sudden belt deceleration can impose large tension reversals into the belt as it attempts to slow the rotor mass. These decelerations commonly occur at transmission gear shifts or "throttle bursts" (i.e. reving the engine while warming the car). In addition to cumulative belt fatigue damage, squeal noise often occurs, especially if the tensioner is forced against its fixed stop due to tension reversal. Due to the torque sensitive nature of the clutch in accordance with the present invention, as soon as the torque load shifts through zero, the clutch will release the couple between the pulley and rotor. The alternator rotor will be free to decelerate independently of the belt under an applied drag or braking torque. The belt will see only a very small tension reversal, the equivalent of the breaking torque. This characteristic will eliminate deceleration sensitivity in such systems
Other objects and advantages of the present invention will be appreciated from the following detailed description, drawings and claims.